This invention relates to the acquisition and processing of data acquired by a logging-while-drilling (LWD) tool during the drilling of a well borehole. More particularly, the invention relates to methods and devices for acquiring data downhole using sensors in contact with the borehole wall, processing the data and transmitting to the surface, in real-time, parameters of the formation penetrated by the borehole as the borehole is being drilled using LWD telemetry.
Modern well drilling techniques, particularly those concerned with the drilling of oil and gas wells, involve the use of several different measurement and telemetry systems to provide petrophysical data and data regarding drilling mechanics during the drilling process. Data are acquired by sensors located in the drill string near the bit and either stored in downhole memory or transmitted to the surface using LWD telemetry devices.
A downhole device incorporating resistivity, gravity and magnetic measurements on a rotating drillstring is known in the art. A downhole processor uses the gravity and magnetic data to determine the orientation of the drill string, and using measurements from the resistivity device, makes measurements of formation resistivity at time intervals selected to give measurements spaced around the borehole. These data are compressed and transmitted uphole by a mud pulse telemetry system. The depth of the resistivity sensor is computed at the surface and the data are decompressed to give a resistivity image of the face of the borehole wall with an azimuthal resolution of 30xc2x0 or better.
Methods using the known apparatus described above methods are limited to making resistivity measurements in the subsurface and fail to address the issue of other useful measurements that could be made using a logging-while drilling (LWD) device. LWD is similar to methods known as measurement-while-drilling (MWD), and any reference herein to LWD is intended to include MWD, as an alternative embodiment.
The devices described above are also limited to measurement devices that rotate with the drill string and do not take advantage of current drilling methods wherein a mud motor is used and the drill bit could be rotating at a different speed from the drill string or wherein a non-rotating sleeve may be available on which substantially non-rotating measuring devices could be located. The present invention overcomes these inadequacies.
The present invention is an apparatus and method of making measurements of a plurality of parameters of interest of the formation surrounding a borehole while a drillstring with a bit at an end thereof is drilling the borehole. In one aspect of the invention, a plurality of selectively non-rotating sleeves are mounted on the drillstring. One or more extendable ribs are mounted on each of the sleeves. Pads are coupled to each rib and sensors are coupled to each pad. When the ribs are extended, measurements of the parameters are made as the drillstring advances through the formation.
In another aspect of the invention, each of a plurality of non-rotating sleeves includes one or more non-extendable (fixed) ribs with pad-mounted sensors coupled thereto. The sensors on the fixed ribs include at least one of a neutron sensor and a density sensor. Other additional sensors may also be used.
In another aspect of the invention an extendable rib and a plurality of fixed ribs are disposed about the outside of a non-rotating sleeve to define a rib set. Each rib of the rib set includes a pad and a plurality of sensors coupled thereto. A plurality of rib sets are mounted on a single non-rotating sleeve, or one rib set may be mounted on each of a plurality of nonrotating sleeves.
In another aspect of this invention, an extendable rib or plurality of extendable ribs are disposed the outside of a subassembly (or sub) that is part of the drill string. As the drillstring rotates the ribs rotate. Each rib contains a pad and a plurality of sensors. The subassembly is provided with sensors that enable the relative position of each rib to be determined with reference to a direction or gravitational orientation.
In another aspect of the invention, the drill bit is mounted on a rotating drillstring and the downhole assembly is provided with sensors that rotate with the drillstring to make measurements of the parameters of interest. The assembly is provided with magnetic, gravitational and/or inertial sensors to provide information on the orientation of the measurement sensors. A telemetry system sends information downhole about the depth of the drilling assembly. A processor downhole combines the depth and azimuth information with the measurements made by the rotating sensors, uses redundancy in the data to improve S/N ratio, compresses the data and sends it uphole by a telemetry system or stores it downhole for later retrieval.
In another aspect of the invention, the drill bit is driven by a downhole drilling motor. The motor may be on a rotating drillstring or on coiled tubing. The sensors for measuring the parameters of interest could be rotating with the drill bit. Alternatively, the sensors could have one of several configurations. In one configuration, the sensors are mounted on a substantially non-rotating sleeve; in another configuration, the sensors are mounted on pads and the pads are coupled to ribs that could be rotating or non-rotating, the pads being hydraulically or mechanically actuated to make contact with the borehole wall. In any of these arrangements, the downhole assembly is provided with sensors that make measurements of the parameters of interest. The assembly is provided with magnetic, gravitational and/or inertial sensors to provide information on the orientation of the measurement sensors. A telemetry system sends information downhole about the depth of the drilling assembly. A microprocessor downhole combines the depth and azimuth information with the measurements made by the rotating sensors, uses redundancy in the data to improve S/N ratio, compresses the data and sends it uphole by a telemetry system. The parameters of interest include resistivity, density, compressional and shear wave velocity and structure, dipmeter, dielectric constant, acoustic porosity, NMR properties and seismic images of the formation.
In another aspect of the invention, the drill bit is adapted to function as a resistivity sensor. A current is generated by a first toroid. The current flows through the tool assembly, drill bit and formation. Current in a second toroid is generated by the current flowing through the tool and a resistivity is determined from current in the second toroid.
As a backup to, or independently of, obtaining the depth information by downhole telemetry, the present invention also provides a capability in the downhole microprocessor to use measurements from sensors at more than one depth to provide a rate of penetration. Surface-measured depths can also be integrated with the measurements from the sensors using a surface mounted depth tracking system on a drilling rig.